Wall of dust engulfs Phoenix

Visibility dropped dramatically in Phoenix as an intense dust storm rolled in [REUTERS]

An enormous wall of dust engulfed the US city of Phoenix on Tuesday evening, paralysing the southwestern city in haze of sand and air-borne debris.

It rolled in at dusk, approximately 8pm local time (3 GMT on Wednesday), followed a few minutes later by a heavy downpour. The rains, the heaviest to hit Phoenix all year, were welcomed in the drought-hit city.

This massive, rolling wall of dust is known as a "haboob", from the Arabic word for wind. Haboobs of the scale of the Phoenix cloud are a daunting sight and can swell to 100km across and 1,000m high.

Such storms are usually caused by the outflow of cool air from a huge, encroaching thunderstorm. The downdraft of the storm-cloud hits the ground where it kicks up a large amount of dust. This cloud then runs ahead of the thunderstorm, enveloping everything in its path.

Within the haboob, visibility is bad and the wind is strong. Inside the Phoenix storm, visibility dropped to 200 metres and a gust of 85 kph was recorded. Another feature of this type of dust storm is often a significant drop in temperature because the outflow of air is from the top of the top of the thunderstorm cloud. As the haboob moved over Phoenix, the temperatures dropped four degrees Celsius in ten minutes.

Haboobs are not uncommon. They occur across the southwestern US, Arabian Peninsula and Northern Africa. Phoenix, the capital of the US state of Arizona, endures an average of three haboobs each year.